runtime: add symbolic constants for freebsd/386 syscalls
Brings freebsd/386 on par with other OSes/archs.
Updates #53479
Change-Id: I16a7d051233e06e589b19919e68f07b6bf92f73c
Reviewed-on: https://go-review.googlesource.com/c/go/+/413365
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Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
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